Significant Customers and Concentration of Credit Risk (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Significant Customers and Concentration of Credit Risk
Revenues	$ 453,538	$ 420,516	$ 415,120	$ 373,273	$ 369,153	$ 280,576	$ 442,822	$ 347,263	$ 1,662,447	$ 1,439,814	$ 1,493,557
Accounts receivable	266,560				187,445				266,560	187,445
Revenues | Customer concentration | First significant customer
Significant Customers and Concentration of Credit Risk
Revenues									184,000	175,100	203,300
Percentage of revenues from a significant customer									10.90%	12.70%	13.40%
Accounts receivable	20,300				12,500				20,300	12,500
Revenues | Customer concentration | Second significant customer
Significant Customers and Concentration of Credit Risk
Revenues									183,800	138,700	96,100
Percentage of revenues from a significant customer									10.90%	10.00%	6.30%
Accounts receivable	45,700				45,100				45,700	45,100
Revenues | Customer concentration | Third significant customer
Significant Customers and Concentration of Credit Risk
Revenues										165,300	297,800
Percentage of revenues from a significant customer										11.90%	19.60%
Accounts receivable					$ 3,900					$ 3,900